In testing the Phenom II X6, Linux Magazine noticed that AMDs new processor doesn't have the full performance under the current Linux kernel. A bug in Linux power management throws a wrench into the six-core processing.

With Ubuntu 9.04 and kernel 2.6.28, the Phenom II X6 1090T processor exhibited 19% better performance, much like Intel's "Gulftown" Core i7 980, in the LUbench benchmark. However, the performance was far more disappointing with Ubuntu 9.10 under kernel 2.6.31. Only by deactivating AMD's Cool'n'Quiet power-saving technology did the Phenom II X6 get anywhere near the expected performance with Ubuntu 9.10.

AMD's Operating Systems Research Center (OSRC) in Dresden, responsible for Linux development under the direction of Chris Schlaeger, confirmed that the new Turbocore mode of the Phenom II X6 was getting into conflict with the kernel's powernow-k8 frequency regulator module. The module falsely assumes the turbo clock as the maximum frequency. But because only the CPU itself can do the overclocking, this leads to a Turbocore knockout. Even a reduction to miminum clocking is impossible. The problem affects all AMD CPUs with the Turbocore feature under kernel 2.6.30 and later. The only available CPUs with Turbocore right now are the Phenom II X6 1055T and 1090T.

AMD has already sent a relevant patch upstream. The chipmaker is encouraging distro maintainers to update with this patch as soon as possible. The patch should be included in the next Ubuntu 10.04 kernel update. To determine if the issue affects your system, use the following command:

dmesg | grep "pstate 0"

Alternatively, use a widget or applet that shows all available processor clock rates.

If the maximum frequency (pstate 0) is shown as 3600 MHz for the 1090T or 3300 MHz for the 1055T, the system's performance is reduced.

Until the patch is available, Linux users should deactivate Cool'n'Quiet in the BIOS, or blacklist the powernow-k8 kernel module (the latter seems a better solution if Windows is also installed).In our tests, the power use is only 17 watts higher when idling, at least under the unaffected kernel in Ubuntu 9.04. No difference was measured under heavy load, whether it was with one core or all six.

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Klaus Knopper is the creator of Knoppix and co-founder of the LinuxTag expo. He currently works as a teacher, programmer, and consultant. If you hav a configuration problem, or if you just want to learn more about how Linux works, send your questions to: klaus@linux-magazine.com